AD7293 datasheet(72/79 Pages) AD

72 / 79 page

AD7293

Data Sheet

Rev. D | Page 72 of 79

MODES OF OPERTION

There are two methods of initiating a conversion on the

AD7293: background mode and command mode.

Background Mode (BG)

The AD7293 can be configured to continuously convert on a

programmable cycle of channels, making it the ideal mode of

operation for system monitoring. These conversions take place

in the background and are transparent to the master. Typically,

this mode is used to automatically monitor a selection of channels

with either the limit registers programmed to signal an out of

range condition via the alert function or with the minimum/

maximum recorders tracking the variation over time of a

particular channel. Reads and writes can be performed at any

time during this mode (the result registers contains the most

recent conversion results).

On power-up, this mode is disabled. This mode can be enabled

by writing to the background enable bits (VINx background enable

sensor background enable register; and the RS+MON, supply

monitor, and BI-VOUTx background enable register) from the

configuration page. The background conversions are active only

when CS is pulled high, that is, the interface is not active.

When CS is pulled low, the conversions pauses and resumes

from the last channel in the cycle when CS is pulled high again.

The user can read back the conversion results via the channel

specific result registers.

If a command mode conversion is requested while the background

mode is active, the scheduled background mode conversion from

the cycle pauses while CS is low and tags onto the command mode

conversion. Conversion is reflected in the ADC busy signal, which

stays true for the combined duration of the command mode

conversion and the background mode.

If the background conversions are enabled during the closed-

loop mode operation, they run continuously irrespective of CS

status. However, the results of the ADC conversions are only

stored if CS is pulled high.

The ADC background cycle prioritizes in the following order:

VIN0 to VIN3, TSENSEINT, TSENSED0, TSENSED1, ISENSE0 to ISENSE3,

voltage supply monitoring, BI-VOUT0MON to BI-VOUT3MON, and

RS0+MON to RS3+MON.

Command Mode

Command mode is useful for controlling the sampling instant

on the VINx channels if an ac waveform is being converted. To

enter this mode and initiate a conversion on a channel, the

special command byte, 0x82, must be written to the device.

When the conversion command is received, the AD7293 uses

the current values in the registers on the sequence page to

determine which channel to convert on and subsequently read

back from. The common result register is updated with the

result of the current conversion channel, which allows the user

to continuously read back the conversion results in command

mode. The ADC command mode sequencer prioritizes in this

order: VIN0 to VIN3, TSENSEINT, TSENSED0, TSENSED1, ISENSE0 to

ISENSE3, voltage supply monitoring, BI-VOUT0MON to BI-VOUT3MON,

and RS0+MON to RS3+MON. The sequencer can be reset by writing

to any of the sequence registers.

In the example in Figure 55, to initiate the continuous conversion

command mode, point to the sequence page and write to the

relevant sequence registers. The ADC sequence register is

programmed to convert on analog input channels, VIN0 to VIN2,

in this example. The first conversion takes place when the AD7293

enters command mode after the special command byte. Every

subsequent conversion is initiated after the result readback

frame, as shown in Figure 55.

Figure 56 shows another example for a command mode conver-

sion with a fixed 24-bit SPI frame length. The first conversion is

initiated when the device enters the command mode after the

special command byte, which is followed by a 24-bit readback

of the conversion result. The device exits command mode

when CS is pulled high, although the sequencer is not reset.

Every subsequent conversion is initiated by reentering the

command mode via the special command byte after which the

user must wait long enough to allow the device to finish any

conversions before reading back the next result.

Current Sensor and Temperature Sensor Conversions

Conversions on the temperature and current sensor channels

can be enabled only via one set of registers, TSENSEx background

enable and ISENSEx background enable, respectively, unlike the

other channels, because the current sense and temperature

sense amplifiers work by integrating the input voltage for a

fixed amount of time, depending on the gain required. At the

end of this integration period, a request is sent to the ADC for a

conversion to be performed. The ADC deals with these requests

in the order in which they arrive. For the other channels, the

ADC starts converting immediately. The corresponding sequence

and current sensor results in the command mode readback

sequence and not to enable conversions on these channels.

AD7293 Datasheet(PDF) 72 Page - Analog Devices

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